In the last decade, modems have become virtually ubiquitous data communications devices in the western world as the use of small, inexpensive general purpose computers has spread into offices and homes. The proliferation of inexpensive computers has led to a like proliferation in the uses to which they are put, including transfers of substantial collections of data via modem over the public switched telephone network. Both technology and the marketplace are driving modem designs toward higher bit rates and modems have become quite price competitive.
In many applications, high data rates are desired, but only needed for short periods of time, or only in one direction of communication over the telephone link. For example, a wide variety of literature is reproduced on mainframe computers and made available for searching to persons doing research in areas to which the literature is pertinent. This use usually requires a relatively small amount of data traffic going from the user's data terminal equipment to the mainframe computer in order to execute the searching instructions to find relevent documents. When the time comes for the user to examine the documents in question, it is often desirable to transfer, or download, substantial blocks of textual data representing the articles themselves. These are often saved to disk and reviewed at a later time when the user is not encountering connect time charges.
In order to fully exploit the available bandwidth of a standard voice grade telephone line on the public switched network, very sophisticated signal processing schemes must be employed. For example, a modem constructed according to CCITT Recommendation V.32 requires complex echo cancelling devices to be employed in order to effectively allow bidirectional, full duplex communication in which transmission in each direction occupies the same frequency spectrum available in the telephone channel.
For a number of years it has been recognized that half duplex modems, in which communication proceeds only in one direction at any given time, have the advantage of allowing the full bandwidth to be available for unidirectional communication. Therefore, for a given available bandwidth and unidirectional data transmission speed, a half duplex modem is normally less expensive than a full duplex modem having bidirectional simultaneous data communications capability at the same bit rate. However, half duplex modems require a handshaking or control scheme to allow the channel to turn around. Many users of publicly available data bases are familiar with the inconvenience of using half duplex modems in which it is not uncommon to encounter a situation where a host computer will commence sending a large block of data which the user would like to interrupt, but is unable to do so until the transmitting modem releases the channel to allow the remote user to send data back to the host.
Additionally, as modems have gone to higher data communications rates, and the costs of powerful microprocessors and other digital electronic components have dropped, it is becoming much more common for modems to employ digital signal processing (DSP) techniques in their implementation. As voice grade telephone line modems are driven to higher speeds, they are likewise driven to data communications protocols with more crowded encoding constellations. It is well known to those skilled in the art that most modern medium speed modems employ communications protocols in which changes in the relative phase and amplitude of a carrier signal are used to encode multiple data bits during any given keying interval or baud time. The baud rate is the fundamental keying rate or symbol rate of a modem. The number of bits encoded per symbol will reflect the density and complexity of the signal constellation. The constellation is graphically represented by a series of points in a Cartesian plane where the angle from a given reference axis (normally the positive X axis) represents a relative phase for a particular symbol and the distance from the origin represents the normalized amplitude of the carrier for the symbol. Since there must be a discrete point in the phase plane for each possible symbol, the number of points in the phase plane increases geometrically with the number of bits per baud. In other words, if N bits are encoded in each symbol, there must be 2.sup.N points in the signal constellation.
Naturally, given noise and frequency limitations of a standard voice grade telephone channel, there is a limit to the range of amplitudes available in a signal constellation. Thus, as the number of bits per symbol increases, the signal constellation becomes more crowded and the distance between neighboring points in the constellation is reduced. As this occurs, there is an increased likelihood of error due to a modem receiver incorrectly decoding a constellation point as being one of the near neighbors of the point actually transmitted.
In recent years, a number of arrangements have been employed to overcome this limitation. The two most common mechanisms are the use of trellis encoding schemes and adaptive equalizers. Trellis coding refers to a genus of encoding schemes which algorithmically and dynamically, based on the sequence of recently sent symbols, change the data significance of particular points in a signal constellation. Simply stated, trellis encoding schemes assure that sequential constellation points actually transmitted by the modem will be separated by a large Cartesian distance in the constellation phase plane. The use of trellis encoding schemes has been mathematically shown to provide an improvement which may be quantified as an equivalent improvement in signal to noise ratio on the communciations channel.
Adaptive equalizers are digital signal processing devices which dynamically adjust phase and amplitude response of a modem's receiver channel in order to compensate for dynamic changes in the overall phase and amplitude transfer function of the communications channel. Simply stated, an adaptive equalizer measures an error signal during the decoding processing and makes adjustments in its own phase and amplitude response to offset the errors in the phase and amplitude of the received signal. In other words, when a received signal at the center of a baud time is decoded to a particular constellation point, the decoding process determines the particular one of the defined constellation points to which the received constellation point is closest. The difference between the phase and amplitude value of the received point and that for the defined point is an error quantity. This information is used to adjust the phase and amplitude response of the dynamic equalizer so that the next receipt of the same signal would be decoded to be exactly (within the limitations of quantitazation error and the like) at the phase and amplitude location of the defined constellation point.
Naturally, the rate at which such parameters are updated in an adaptive equalizer is selected to give the best tradeoffs in actual operating characteristics. Like any multiple order feedback device, too much gain in the equalizer will tend to cause its characteristics to exhibit the characteristics of an under damped mechanical system in which there is overshoot and undershoot in searching for the correct point. Too slow a response time for the adaptive equalizer may be likened to an over damped system in which the equalizer will respond too slowly to compensate for changes in the transmission channel transfer function.
This leads to an understanding of one of the main drawbacks of high density constellations employed in modem communications. All of the digital processing devices, and in particular adaptive equalizers, require a finite amount of time in which to adjust to the signal conditions actually encountered on the line. In practice, it is common to initialize or train such devices by predetermined rules for sending known bit sequences so that the DSP elements will become adapted to the communications channel actually being used. This is normally referred to as a training sequence. The time it takes a given DSP receiver to become properly locked on to the conditions of the channel is referred to as the training time.
Putting all of the constraints and motivations described hereinabove together will lead to a full understanding of the following. In order to make relatively low cost high speed modems, particularly suitable for applications in which high speed communication is normally required in only one direction, several manufacturers have introduced non-standardized communications protocols. The most popular in the United States is that employed by the Hayes V-Series Smartmodem 9600 brand modem. This particular modem is a fast turnaround half duplex 9600 bit per second device which employs the signal constellation and the trellis encoding arrangement defined in CCITT Recommendation V.32, which Recommendation pertains to full duplex 9600 bit per second modems. Significant inventive work was required in order to design a device which could turn the channel around fast enough, and have the required training sequences for the DSP receivers to be of short enough duration so that the modem would, in many pratical applications, appear to be full duplex to the user. The basic design of the Hayes V-Series Smartmodem 9600 brand device which allow it to achieve these operating goals are described in U.S. Pat. No. 4,894,847, issued Jan. 16, 1990 entitled "High Speed Half Duplex Modem With Fast Turnaround Protocol" assigned to the assignee of the present invention. Said U.S. Pat. No. 4,894,847 is hereby incorporated by reference.
Another arrangement which has been employed is that of a full duplex asymmetrical modem. While generically such devices have been known for a number of years, the HST modems manufactured in the United States by U.S. Robotics Corporation are an example of this type of device. An asymmetrical modem is not a true half duplex modem but one in which there is a relatively high speed channel going in one direction and a relatively low speed channel going in the other, for example 9600 bits per second and 300 bits per second, respectively. The U.S. Robotics HST modem is designed so that the directional significance of these channels may be turned around at a relatively high speed, thus dynamically changing which communicator on the telephone link has the high speed channel and which has the low speed channel.
In order to overcome the constraints of relatively long training times in DSP implemented modems, it is known to those skilled in the art to freeze, or terminate updating of certain dynamically adjusted operating parameters in the digital signal processing path. U.S. Pat. No. 4,621,366 to Cain et al. shows a DSP modem which stores previously acquired parameters and coefficients when the channel is turned around, and upon the reestablishment of operation of the receiver, resets the DSP equalizer with the stored coefficients and provides a shortened retaining sequence. This arrangement allows for shortened training times after channel turnaround since the stored coefficients are based on data derived from the physical attributes of the communications channel which are very unlikely to have changed significantly in the time which elapses between successive operations of the receiver.
Also, it is known in the art to, upon a given set of conditions, terminate the updating of adaptive equalizer parameters. This approach is predicated on the assumption (normally a good one) that the adaptive equalizer will have learned something about the actual physical channel over which it is communicating during the time it is operated. Rather than reset the equalizer parameters to zero when the receiver is turned off, for example when a channel is turned around in a half duplex modem, the modem will maintain the coefficients which define the operating parameters of the equalizer once data reception has terminated. When the channel again turns around, the adaptive equalizer will still be in the same state at the time its receiver was cut off. U.S. Pat. No. 4,669,090 to Betts et al. shows such a modem. Operating the receiver in this manner causes the equalizer to start from a known set of coefficients. These coefficients will be very close to the actual ones it will need as it again commences the receipt of data since these coefficients are based on its experience with the characteristic of the telephone communication channel currently in use, and these characteristics typically change at a slow rate.
However, there are a couple of potential drawbacks which have to be offset in employing such an arrangement. First, by its very nature, the termination of operation of parameter update in an adaptive equalizer halts the flow of data through the device. In other words, there is no way to terminate the operation of the adaptive characteristic of the equalizer and still pass data through it. Therefore, it is necessary to provide an additional period of received signal, over and above that containing the data actually being received, in order to allow the received data itself to pass through the equalizer and on through the other components of the DSP signal path, to the point at which it is finally decoded and passed on to a utilization device. In the present specification, the additional period of the received signal will be referred to as the "flush time" which is required to clear the useable received data through the digital signal processor.
As is known to those skilled in the art, the transmitted signal for the flush time is necessary both to flush the informational received bits from the digital signal processing path as well as to give a stream of "real samples" to the adaptive equalizer so that it will not simply start responding to the noise on the line when the carrier signal is terminated. In other words, if one simply turns off the carrier after the last informational bit is sent, the DSP path and the receiver will have to continue to operate for at least the flush time in order to make sure that all of the informational bits pass through the receiver and on to the utilization device. When this is done, the adaptive equalizer will have started to respond to the noise in the absence of carrier when the channel is being turned around, and the dynamically adjustable parameters will move quickly to values which are quite different from those which were established during operation of the receiver while a carrier signal was on the line.
The transmitter of the modem described in the above referenced U.S. Pat. No. 4,894,847 has been designed to provided to provide a 15 millisecond flush time of continued carrier on the channel after transmission of the last data bits before the carrier is cut off and turnaround of the channel commences. However, the flush time simply adds to the turnaround time for the channel in a half duplex fast turnaround mode. If the flush time is made arbitrarily long, then the turnaround time becomes unacceptably long.
Therefore, it will be appreciated that there is a problem in the design of high speed turnaround half duplex modems employing DSP receivers which arises in two contexts. There is the need to provide additional carrier on times after transmission of the last useable data bits in a given direction in order to maintain the adaptive equalizer in an operating condition which responds to the actual telephone communication channel in order to be able to freeze these parameters when the channel is turned around. As noted above, the modem of the above cited U.S. patent uses a 15 millisecond flush time in order to flush the DSP in the receivers of like modems. The present invention arose in the context of an improved DSP receiver for use in a modem of the type described in the 4,894,847 patent. Certain improvements to the DSP receiver led to and end-to-end delay time between the analog input to the receiver and its final decoded output which was in excess of 15 milliseconds. Thus, the 15 millisecond flush time of the already established communciations protocol was insufficient to assure maintenance of the dynamic operating characteristics of the DSP receiver in accordance with the statistics of the telephone line in use.
Therefore, the need for the present invention arises in the environment of any modem in which it is desired to freeze or maintain dynamic operating parameters of the digital signal path when a receiver is turned off. The most common application in which this condition is encountered is in a fast turnaround half duplex modem. The need arises when either there is no particular arrangement made for continued carrier to provide a flush time, or, as in the case of the necessity which was the mother of the present invention, one in which there was an existing established communications protocol setting a flush time of continued carrier, which flush time was less than the end-to-end delay time of the DSP receiver apparatus.
It will be quickly appreciated by those skilled in the art that these are simply two aspects of the same generic problem: how to keep the adaptive equalizer parameters responding to actual conditions on the telephone link after the termination of transmission of informational bits prior to turning the channel around. The problem must also be understood in the context of the unacceptability of providing and arbitrarily long extension of carrier which will slow down the turnaround time to an unacceptable degree.
Thus, there is a need in the art of digital signal processing modems to provide apparatus which can maintain the dynamic operating parameters of an adaptive equalizer in a state which is based on the actual transfer function of the communication link at a time when the receiver is being turned off, and allow such parameters to remain fixed until the next time the receiver is activated.